1. Field of the Invention
The present invention relates to a wide field high optical efficiency display device, in which a collimated light image is observed superimposed on the vision of the external landscape. The collimating optics reflect to infinity the light image corresponding to the data to be collimated; this results in an absence of effort of accomodation for the eye of the observer and great visual comfort. In the conventional head-up display on board aircraft, the light image is reflected by a combination optical system to the observer. This optical system is traversed by the radiation coming from the external landscape. Thus, the observer sees the collimated light image corresponding, for example, to a synthetic image which conveys air navigation data superimposed on the landscape.
Recent techniques in this field of head-up collimators have been directed essentially towards increasing the instantaneous field of vision of the collimated image, as well as towards obtaining an improved optical efficiency.
2. Description of the Prior Art
One known solution consists in using a spherical mirror on the axis which also limits the optical aberrations. A solution of this kind is described in the French patent application No. 2,542,459; in this embodiment, the optical axis of the spherical mirror corresponds to the normal axis of vision of the observer whose eye is placed at the center of the mirror. This latter is combined with a semitransparent flat mirror whose purpose is to reflect the optical axis of the light image generator to the spherical mirror on the concave side which produces the collimation and sends back the collimated image towards the observer. To increase the efficiency, a holographic spherical mirror is used which reflects the wave length corresponding to the image generator formed generally by the screen of a cathode ray tube. The main drawback of this solution is that the field, although enlarged, remains however limited to values of the order of 30.degree. to 40.degree.. For a greater increase of the field it would be necessary to use the spherical mirror out of the axis for the partially transparent flat mirror would be too close to the eye of the observer. The result would more especially be optical aberrations and difficulties of implementation.
In another solution, mentioned in Optical Engineering September/October 1985/Vol. 24-No. 5/pages 769-780-article entitled "Holographic Mirrors", a first flat semitransparent mirror is used for reflecting the axis of the light image generator to the eye of the observer. The corresponding radiation reflected by this mirror passes through the spherical mirror, which is followed by a second semitransparent mirror perpendicular to the optical axis of the spherical mirror corresponding to the normal direction of vision. The use of this second flat semitransparent mirror is made necessary because of the presence of polarizers for polarizing the radiation of the light image and for then recovering it after collimation by means of the spherical mirror. Thus a selection of the path of the landscape and that of the image formation is obtained as a function of their polarization (so called "pancake" system). This solution increases the field but does not perform at all well from the photometric point of view and raises problems for use in the real situation. In fact, the transmission over the optical path of observation of the landscape is limited to 6.2% and that over the optical path of observation of the synthetic image is limited to 1.6%. This results in losses introduced at the mirrors during multiple reflections and transmissions and by passing through polarizers. It is not possible to use a hologram, since the system is completely along the axis the paths are not separable by holography.